At a web design conference last year, a guy who worked with Amazon gave a presentation and I learned that even selling at cost, Amazon can turn a tidy profit. They roll over their inventory so quickly (I believe it was every 20 days) that there is a significant spread between the time they take your money and the time they pay their bills for the products they sell. During this time, the money is moved to interest bearing accounts and they make a boat load. Rinse and repeat.
Another neat fact was the ratio of product purchases to product reviews is 1,300 to 1. So for a product to have 20 reviews it typically takes about 26,000 purchases. The vast majority of people don't write reviews. I know I never do.
I don't know. I just find this all very fascinating.